The present invention relates to a positioning technique in magnetic resonance imaging, and in particular, to the positioning technique applying multiplanar reconstruction (MPR) to a three-dimensional image data composed of a plurality of tomographic images collected at different slice positions of an object being diagnosed.
In magnetic resonance imaging, an object to be diagnosed is placed in a static magnetic field, whereby atomic nuclei align themselves with the static magnetic field. Then gradient magnetic fields in three x-, y- and z-directions are applied to the object for spatially encoding and a radio frequency (RF) signal is applied to the object for exciting the atomic nuclei in a magnetically sliced plane, which has a certain thickness in a slicing direction, of the object. When the RF signal is removed, magnetic resonance (MR) signals emitted from the sliced plane can be collected. A series of the excitation and MR signal acquisition is performed on a predetermined pulse sequence. The collected MR signals are then processed, for example, by Fourier transformation to form image data of the magnetically sliced plane of the object.
Prior to the scan for diagnosis, it is usual to perform a preparation which includes a process of positioning. There are two ways for the positioning; one way is to use a light localizer, and another way is to use one or two pilot images.
In positioning with a single pilot image, as shown in FIG. 1, a single tomographic image IMi (for instance, an axial image of an object) is given as a single pilot image. On the pilot image IMi, a linear ROI(region of interest) Ra is placed to specify an arbitrary linear position thereon, thus specifying a slice plane perpendicular to the pilot image IMi at its linear position specified by the ROI Ra. Slice positions to be scanned are determined such that they are parallel to the specified slice plane. As a result, a scan for diagnosis is carried out at the determined slice positions. Hereinafter, the term "linear ROI" is used for specifying a linear position, so this includes line ROIs and elongated rectangular ROIs.
On one hand, in positioning with two pilot images, as shown in FIG. 2, the first and second tomographic images IMi and IMj (for instance, both are axial images parallel to each other) are given as two pilot images. Then, one linear ROI Rb is placed on the first pilot image IMi and another linear ROI Rc is placed on the second pilot image IMj. In consequence, a slice plane passing through the two linear ROIs Rb and Rc at the same time can be specified for a scan. Using the two parallel pilot images enables positioning in any direction.
However, it is impossible for the above positioning ways with one and two pilot images to provide an entire tomographic image of a slice plane to be scanned before the diagnostic scan. Hence, there is no means for operators, such as doctors, who place the above linear ROIs to Judge whether or not the slice plane determined by the ROIs catches properly a desired lesion to be examined. In other words, for images except one or two pilot images, operators have to place one or two ROIs with his or her intuition or presumption.
Therefore, improper positioning situations have often been occurred; for example, the position of a lesion is different or deviates from a desired slice plane, for which a diagnostic scan will be carried out. In such cases, most of the images finally obtained will be useless and it is required to repeat the same process of positioning. This results in reduced efficiency of diagnostic throughput.
Further, operators are necessarily required to have a skilled technique and much experience for placing ROIs. So the positioning operation becomes a hard work.
Still further, although the slice position to be scanned can be shown by the two ROIs on the two pilot images, the slice angle of a slice plane are not easily recognizable thereon. As a result, it is difficult to designate a proper slice plane having a desired slice angle.